This most certainly is not a stupid question. FOG did a nice write up somewheres about the differences in construction of the 2. A radial tire is a much better tire. I holds the road better, tracks better, and is just an all around better tire. Not to say that the Bias Ply is a horrible tire, as it isn't. Both tires are capable of extreme lean angles, but the radial tire more so. You should never mix a radial tire with a bias ply tire, as the bike hill handle like a drunken camel with a hinge in the middle. Also, you should never mix different tire brands regardless of whether or not both tires are radial or bias ply.